Wow....
I'm not an expert with iMovie either (used the old '06 version pretty extensively but not so much the recent editions). I did, however, encounter some very odd issues a co-worker had with her iMac (one of the first generation of Intel CPU white iMacs) using iMovie and iDVD.
Basically, she was having issues where projects in her iMovie wouldn't import properly into iDVD, despite repeating the exact same process she had always used successfully in the past. Even if you tried to simply drag and drop one of the files onto an iDVD template on the screen, it would act like it wasn't a valid file to drop in a drop zone.
As she was still on OS X 10.4 Tiger, I convinced her it was probably worthwhile to upgrade to Snow Leopard and iLife '11. Well, I did the Snow Leopard upgrade, which appeared to go fine, and followed up with a new install of iLife '11. After that, her machine started randomly freezing up with the spinning beach ball, to the point it was almost unusable!
I ran Disk Utility and did a repair permissions, but it found nothing of note to fix. I started suspecting bad RAM at that point so put new DIMMs in it. No help! Finally, I tried the "PRAM reset" procedure, holding down CMD-OPTION-P-R until it chimed several times in a row. I really didn't expect this to do anything useful but I was out of ideas!
Well, amazingly - all the freezes stopped. I let the machine run all night long doing a Time Machine backup and it was still fine the next morning! Then I tried out the iMovie '11 and iDVD combination and that worked properly too, EXCEPT for a weird issue with iDVD complaining that one of the templates was missing a file. I deleted iDVD out of the Applications folder and reinstalled from the iLife '11 disc again. Problem solved and the iMac has been working fine for her for several weeks now!
So what's the point in me telling you all of this? I guess only that I've learned some of these odd issues can turn out to be caused by relatively minor stuff (like needing to do a PRAM reset!), and simply deleting the troublesome app and reinstalling a fresh copy again. I don't know exactly where iMovie '11 keeps its configuration files either, but I'd look inside the Library folder inside your home directory for those and delete them along with the app itself, before reinstalling, to make sure the whole thing is reset.